Subject: Re: Incredible!
From: Timothy A. Seufert (tas@mindspring.com)
Date: Wed Sep 12 2001 - 15:03:55 MDT
At 4:35 PM -0400 9/12/01, Robert Vogt IV wrote:
> Alex,
>
>> I just got through running an alpha of the modelling program I am
>> working on, under both RedHat and YDL, and I don't understand it. Could
>> someone who is technically minded, explain to me why this G4, Dual
>> Processor 800 MHz, goes about 15 times faster then the Dual Processor
>> Pentium 4, 1.8 GHz. Both machines have 1.5 Gigs of Memory. I'm a
>> mathematician not a computer guru. And my math skills say that 1.8
>> GHz >> 0.8 GHz.
>
> Very briefly (I'm in class) - the G4 is RISC chip. A RISC chip
>contains no microcode decoding unit, and therefore can dispatch
>instructions up to 10x faster.
Can you give even *one* example of a modern RISC processor that
dispatches instructions ten times faster than a modern x86?
Word to the wise: don't waste your time looking. You won't find one.
AMD's K7 core has an aggressive decoder design that decodes and
dispatches up to three x86 instructions per cycle. You will have to
find a microprocessor that dispatches 30 instructions each cycle to
meet your claim. Good luck finding one that does even 10. Modern
PowerPCs dispatch two (G3, G4) or three (G4+) per cycle.
More words to the wise: It's not having to look up microcode
sequences that makes x86 instruction decode difficult.
Finally, PowerPC implementations either have microcode for or have to
rely on exception based emulation of certain instructions, because
there are a couple whoppers which definitely do not fit the
traditional RISC model (such as string copy instructions).
>Additionally, the G4 has the AltiVec
>engine...
And the P4 has SSE2.
The most likely reason for the performance oddity seen above is
simply that the program in question runs into one of the performance
pitfalls seen in the P4. For example, if it does a ton of bitwise
rotation, the PPC will beat it easily.
It's unlikely that AltiVec is a factor, because almost no Linux
software uses AltiVec. Who knows, maybe this modelling program is
the exception, but even on MacOS AltiVec support is not really
widespread.
-- Tim Seufert
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